Power can be provided to a LAN device such as an Ethernet LAN connected telephone (often referred to as an IP telephone) through spare-pair wires, for example, through the 7/8 pair of wires in an RJ-45 jack configuration. Alternatively, power can be supplied by phantom-feed powering (or by both phantom-feed and spare-pair powering).
When power is supplied by phantom-feed, power is fed from a LAN hub through the transmit and receive signal lines to the LAN device. For phantom-feed powering, autotransformers are used on the secondary side of the voice over Internet protocol (VOIP) transformer to prevent phantom-feed current from going to the main transformer.
Prior art LAN transformers do not include an autotransformer and are therefore not suitable for use in a phantom-feed LAN. On the other hand, prior art VOIP transformers are provided with built-in autotransformers. Such a prior art VOIP transformer is suitable for use with a phantom-feed LAN since phantom-feed current is prevented from going to the main transformer by the autotransformer. However, VOIP transformers (Ethernet transformers with powering capabilities) are more expensive than conventional transformers. Further, many devices are not designed for phantom-feed powering. Many devices are only functional with spare-pair powering and are not functional in a phantom-feed LAN. Also, devices designed for both phantom-feed and spare-pair capabilities are more expensive relative to devices designed only for spare-pair powering.
It is therefore desirable to provide an apparatus for providing power for a LAN connected devices (e.g. telephone) designed to use spare-pair powering but connected to a phantom-feed LAN.